Where is telephone country code 00373?

Telephone country codes never begin with zero.

Country code +373, dialed as 00 373 from many places, is Moldova (in eastern Europe).

Country code +7, dialed as 003 7 from many places, is Russia and Kazakhstan. A number beginning with +7 3 is in Russia.

To avoid confusion, it is best to write an international telephone number in correct international format, beginning with the plus symbol and the correct telephone country code (e.g., +373 or +7), omitting any dialing prefix.

(The plus signmeans "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)